1. Field of the Invention
The present invention relates to a light path shift device, which is used in a display device and an image input device, for performing light path control with a light path shift unit for shifting a light path, and an image display device using the light path shift device.
2. Description of the Related Art
In the following, a light path shift unit indicates an optical element which deflects a light path of light based on an external electric signal, namely, it is an optical element which shifts the emitting light relative to the incident light in parallel, or rotates the emitting light at a certain angle, or combines these two operations to change the light path. In addition, the magnitude of the light path shift in the parallel shift is referred to as “shift distance”, and the magnitude of the rotation of the light path in the rotational shift is referred to as “rotational angle”. Additionally, it is assumed that a light path shift device includes such a light path shift unit and is able to shift the path of the light.
Further, in the following, it is assumed that a “pixel shift device” includes an image display unit having plural two-dimensionally arranged pixels each able to control light passing therethrough according to image information, a light source for illuminating the image display unit, an optical member for observing an image pattern displayed on the image display unit, and an optical deflection unit for deflecting a light path between the optical member and the image display unit for each of plural sub fields obtained by dividing an image field in a time division manner. In the pixel shift device, the optical deflection unit deflects the light path for each of the sub fields, such that image patterns at shifted positions, which correspond to the shift of the light path of each of the sub fields, are displayed on the image display unit, and as a result, it appears that the number of pixels is doubled on the image display unit.
In the related art, by applying a transverse voltage on a homeotropically-aligned Chiral Smectic C liquid crystal to change a tilt angle of the liquid crystal molecules, and along with that, due to a change of the birefringence, the light path of the incident light can be shifted.
In addition, in a pixel shift unit using the homeotropically-aligned Chiral Smectic C liquid crystal, by a voltage application unit (a line electrode) and a transparent resistor, a transverse voltage is applied uniformly.
When using the pixel shift unit in a display device, the pixel shift unit is arranged between an image display unit, such as a liquid crystal image display device, and an optical system for magnifying the image from the image display unit. When the pixel shift unit shifts the light path by half pixel pitch in totally four directions, sub-images are created by extracting contents from an image to be displayed on one screen every two pixels in the vertical direction and the horizontal direction, and in response to the light path shift operation of the pixel shift unit, four sub-images are sequentially displayed on the image display unit. In this way, with an image display unit having relatively a small number of pixels, it is possible to display images of high resolution. Namely, it is possible to display images each including the number of pixels of the display unit multiplied by the shift level numbers.
FIG. 14 is a schematic view of a configuration of a magnification display device using a light path shift unit.
Shown in FIG. 14 are a light source 81, fly-eye lens arrays 82, 83, a condensing lens 84, a projection lens 85, a screen 66, a control circuit of the liquid crystal panel 88, a light path shift unit 89, a control circuit 90 of the light path shift unit 89, a polarized beam splitter 91, and a reflection-type liquid panel 92 serving as the image display unit.
The fly-eye lens arrays 82, 83 are integrator optical systems for homogenizing the light from the light source 81. The condensing lens 84 condenses the light onto the image display unit 92 for illumination. The polarized beam splitter 91 splits the illumination light and the imaging light.
The light emitted from the light source 81 is homogenized by the fly-eye lens arrays 82, 83, which serve as the integrator optical systems. The condensing lens 84 converts the incident light into nearly parallel light to illuminate the liquid crystal panel 92. The light path shift unit 89 shifts the imaging light by a preset distance along the pixel arrangement direction. The light is magnified by the projection lens 85, and is projected onto the screen 66.
It is preferable that the shifted distance be one part of integral multiple divisions of the pixel pitch. For example, when it is desired to double the pixel numbers along the pixel arrangement direction, the shifted distance may be half of the pixel pitch; when it is desired to increase the pixel numbers four-fold along the pixel arrangement direction, the shifted distance may be a quarter of the pixel pitch.
In either case, plural sub fields are created by dividing an image field in a time division manner according to the shift level number, the pixel shift unit operates for each sub field, and images are displayed on the display unit at positions corresponding to the states of the pixel shift unit. In this way, it is possible to apparently display images of high resolution.
In the above, the reflection-type liquid panel 92 as shown in FIG. 14 is used as an example for illustration; certainly, a transmission-type liquid panel, a micro mirror, or other two-dimensionally arranged optical elements can also be used as the display unit.
FIG. 15 is a schematic view of a basic configuration of the light path shift unit illustrating light paths of the light path shift unit.
Shown in FIG. 15 are a light path shift unit 1, transparent substrates 2, 3, an alignment film 4, and a ferroelectric liquid crystal 5 including a chiral smectic C phase.
The alignment film 4 is formed on an inner surface of at least one of the transparent substrates 2, 3, and the ferroelectric liquid crystal 5 including a chiral smectic C phase is supplied between the alignment film 4 and the other transparent substrate. In the smectic liquid crystal, liquid crystal molecules are arranged in layers along the long axis direction. When the normal direction of the liquid crystal molecule layer is in agreement with the long axis direction of the liquid crystal molecules, the liquid crystal in this state is referred to as “smectic A phase”; when the normal direction of the liquid crystal molecule layer is not in agreement with the long axis direction of the liquid crystal molecules, the liquid crystal in this state is referred to as “smectic C phase”. In the smectic C phase, when the external electric field is not in action, the direction of the liquid crystal director of each liquid crystal molecule layer is spiraled. The chiral smectic C phase has a molecular structure including asymmetric carbon atoms; thereby, spontaneous polarization occurs. Because the spontaneous polarization occurs, the liquid crystal molecules are rearranged in the direction determined by the spontaneous polarization Ps and the external electric field E, and in this way, optical properties of the liquid crystal molecules are controlled.
The chiral smectic C phase ferroelectric liquid crystal 5 constitutes a homeotropic alignment, namely, due to the alignment film 4, the chiral smectic C phase ferroelectric liquid crystal molecules are aligned in a molecular spiral rotation manner with the rotational axis of the molecular spiral rotation being perpendicular to the substrate. In the light path shift unit 1, corresponding to the light deflection direction, a not-illustrated electrode pair are arranged on the front side and the back side of the paper, and this electrode pair is arranged so that the electric field vector is nearly perpendicular to the liquid crystal rotational axis of the light path shift unit 1.
In addition, compared to smectic A phase or nematic liquid crystal, the chiral smectic C phase liquid crystal 5 is capable of response at very high speed, and is capable of switching on the order of sub ms. Especially, since the direction of the liquid crystal director is uniquely defined relative to the electric field, it is easy to control the direction of the liquid crystal director compared to smectic A phase liquid crystal.
FIG. 16 is a schematic view illustrating the liquid crystal alignment in the light path shift unit shown in FIG. 15.
In FIG. 16, the electric field is applied in a direction perpendicular to the paper. Liquid crystal directors 8 are shown in FIG. 16. A Cartesian coordinate system is established as shown in FIG. 16, and in this coordinate system, in an XZ cross section in the liquid crystal, the liquid crystal directors 8 are distributed as shown in FIG. 16.
FIG. 17 is a longitudinal sectional view of the portion in FIG. 16.
In FIG. 17, θ represents a tilt angle of the liquid crystal director relative to the rotational axis of the liquid crystal. In the following, it is simply referred to as a “tilt angle”. It is assumed that the spontaneous polarization Ps of the liquid crystal is positive, and an electric field E is applied along the +Y direction (pointing to the paper). So, the liquid crystal director is in the XZ plane since the rotational axis of the liquid crystal is nearly perpendicular to the substrate.
It is assumed that the refractive index in the long axis direction of the liquid crystal molecules is denoted as “ne”, and the refractive index in the short axis direction of the liquid crystal molecules is denoted as “no”. Then, if the incident light is a linearly-polarized light beam having a polarization plane in the direction of the Y-axis, and the incident light propagates in the +X axis direction, the incident light acts as ordinary light in the liquid crystal, and is refracted with the refractive index of no; but the light beam perpendicular to the incident plane propagates directly in the direction “a” as shown in FIG. 17, namely, the light beam perpendicular to the incident plane is not refracted.
On the other hand, when the incident light is a linearly-polarized light beam having a polarization plane in the direction of the Z-axis, the refractive index of the incident light is determined from the direction of the liquid crystal director, and from no and ne. Specifically, the refractive index of the incident light is determined from the relationship with the direction of the light passing through the center of a refractive ellipsoid with principal axes having refractive indexes no and ne, respectively. The details of determining the refractive indexes of the incident light are omitted.
The incident light perpendicular to the incident plane is deflected according to no, ne, and the tilt angle θ of the liquid crystal director in a direction “b” as shown in FIG. 17, that is, the incident light is shifted. If the liquid crystal thickness (or referred to as “gap”) is represented by d, it is known that the shift distance S can be expressed by the following equation (1).S={[(1/no)2−(1/ne)2] sin(2θ)×d}/{2[(1/ne)2 sin(2θ)+(1/no)2 sin(2θ)]}  (1)
When the electric field is inverted, the liquid crystal director has a linear symmetric arrangement with the X axis as a center, as shown by the dot-dashed line, and the linearly-polarized light beam having a polarization plane in the direction of the Z-axis propagates in the direction “b′”, as shown in FIG. 17.
Hence, by controlling the direction of the electric field, the linearly-polarized light beam can be shifted to positions “b” and “b′”, corresponding to a shift distance equaling 2S.
If the obtainable light deflection is calculated relative the typical property parameters of the liquid crystal (no=1.6, ne=1.8), it is found that when the tilt angle θ of the liquid crystal director is 22.5°, in order to obtain a shift distance 2S=5 μm, it is found that the thickness of the liquid crystal can be 32 μm.
In addition, in a homeotropic alignment ferroelectric liquid crystal, it was reported that a response speed of 0.1 ms was obtained with respect to an electric field of 700 V/cm. For example, please refer to “Ozaki et al., J. J. Appl. Physics, Vol. 30, No. 9B, pp. 2366-2368 (1991)”. That is, a response speed of the order of sub ms is obtainable.
Generally, an image display device such as a liquid crystal panel has an interconnection electrode structure so that line electrodes and column electrodes are arranged in a matrix shape.
FIG. 18 is a schematic plan view illustrating a liquid crystal panel serving as an image display device.
Shown in FIG. 18 are a matrix electrode 924, a line driver 922, and a column driver 923. At the cross portions of the line electrodes and the column electrodes, active elements including transistors are provided, respectively, and by switching the active elements, voltages are applied on the liquid crystal. There are various kinds of methods of displaying images on the liquid crystal panel; for example, generally, it is known that are a “one time rewrite method” and “scanning method”. In the one time rewrite method, the whole screen is rewritten at one time; whereas in the scanning method, the displayed image is rewritten one line by one line sequentially from the top of the screen (line sequential manner).
In the one time rewrite method, frequently, the grade level is controlled digitally, and in this case, the light path shift unit operates in synchronization with the rewriting of the screen; thereby, the method is capable of high definition display.
In the line sequential method, frequently, the grade level is controlled in an analog manner. For example, one screen is rewritten while scanning at a frequency of a few tens of Hz to 300 Hz.
In the one time rewrite method, the light path shift unit may operate at a lump, and it can be realized by using a very simple mechanism.
For example, Japanese Laid Open Patent Application No. 7-64048 (below, referred to as reference 1) discloses a technique in this field.
In the one time rewrite method, the digital grade level is performed by using a ferroelectric liquid crystal, however, the digital grade level control requires a very high speed to transfer data to the image display device. This is a problem especially when using a light path shift technique because this technique needs to further divide frames and to operate at a high frame rate, and the limit on the transfer speed further causes a limit of the grade level number and the number of the displayed pixels.
On the other hand, when writing image data of analog grades by scanning, although there are few limits to the transfer speed, if the light path units are operated at the same time, one image corresponding to one sub-field is divided into portions at different shifted positions, and is displayed with the divisions being at different positions. This is because while the image display device operates in a scanning manner, the light path shift unit of the related art shifts at one time. Further, although the response of the light path shift using the chiral smectic C phase liquid crystal is high, the delay caused by the light path shift is not negligible compared to the time required for one field of one image. FIG. 19 explains this problem.
FIG. 19 us a diagram illustrating a relationship between an electrode scanning position and a shift distance.
As shown in FIG. 19, while the graph representing the shift distance has a trapezoidal shape, the inclined side of the trapezoid is attributed to a response delay relative to level change of the input signal.
As shown in FIG. 19, a sub-field, which originally corresponds to image data to be displayed at one position, is divided into a part corresponding to timing before the operation of the light path shift unit and a part corresponding to timing after the operation of the light path shift unit.
Specifically, up to time t1, the image at lines from a1 to b1 is displayed by the light along a light path b shifted by a distance S; from the time t1 to the time t2, the image at lines from b1 to c1 is displayed by the light along a light path b and sequentially sifted to a light path b′. In other words, the image from a shift distance S toward a shift distance −S is partially moved; from the time t2 to the time t3, the image at lines from a1 to c1, and the image of the subsequent scanning up to a line d1 are displayed by the light along a light path b′ shifted by a distance −S; further, from the time t3 to the time t4, the portion of the image at lines from b1 to d1 is moved from a light path b sequentially shifted to a light path b; at last, after the time t4, the portion of the image at lines from c1 to d1 is displayed by the light along the light path b shifted by a distance S. Namely, the sub-field A is divided into three parts, and they are displayed at two shifted positions.
Next, for the sub-field B, up to time t3, the portion of the image at lines from a2 to b2 is displayed by the light along the same light path b′, part of the sub-field A and part of the sub-field B are combined and displayed at the same shifted position. This causes partial movement of an image and degradation of image resolution.
In addition, as disclosed in reference 1, a ferroelectric liquid crystal is used as a polarization control unit, and at the exit side, a birefringence medium having an inclined crystal axis is arranged. The polarization plane of the polarized light incident on the birefringence medium is modulated in an inclined direction and the crystal axis or a direction perpendicular to the crystal axis, thereby performing light path shift. In the method disclosed in reference 1, relative to the above-mentioned scanning type display unit, an electrode of a polarization direction control panel of the light path shift unit is divided into plural regions perpendicular to the scanning direction; one pair of the electrodes are selected in synchronization with the scanning of the display device and a voltage is applied on the pair of the electrodes. By changing the pair of the electrodes, the light path shift unit is partially operated.
However, for the vertically aligned ferroelectric liquid crystal, there is a basic difference in operating method, namely, the liquid crystal is operated in a direction parallel to the substrate. The method disclosed in reference 1 is not applicable.